This invention relates to a slow positron beam generating device supplied with accelerated particles for generating a slow positron beam due to a nuclear reaction process.
Generally, a slow positron beam generating device of the type described comprises a target member having an incident surface to be irradiated with accelerated particles produced by a particle accelerator, a moderator, and an ejecting electrode. When the incident surface of the target member is irradiated with the accelerated particles, nuclear reaction is caused to occur to thereby generate .beta..sup.+ decay radioisotopes in the target member. The .beta..sup.+ decay radioisotopes emit fast positrons (namely, high energy positrons) in every direction. The moderator receives and moderates the fast positrons to emit slow positrons (that is, low energy positrons). Supplied with the slow positrons, the ejecting electrode ejects a slow positron beam.
Use is made of a different target member having a dual function. In the first place, when the incident surface of the different target member is irradiated with the accelerated particles, the different target member produces the .beta..sup.+ decay radioisotopes due to nuclear reaction within the target member so that the .beta..sup.+ decay radioisotopes emit the fast positrons. In the second place, the different target member moderates the fast positrons to emit the slow positrons.
In a conventional slow positron beam generating device, among the fast positrons emitted in every direction, the fast positrons emitted from the incident surface of the target member are moderated by the moderator into the slow positrons which are ejected by the ejecting electrode as the slow positron beam. When the different target member is used, the slow positrons emitted from the incident surface of the different target member are ejected by the ejecting electrode as the slow positron beam. Thus, in the conventional slow positron beam generating device, either the fast positrons or the slow positrons emitted from the incident surface are used in ejecting the slow positron beam. Therefore, such a slow positron beam generating device is called a reflection type in the art. Such a reflection type slow positron beam generating device is disclosed, for example, by T. S. Stein et al in Rev. Sci. Instrum., Vol. 45, No. 7, July 1974, pages 951-953 (published by the American Institute of Physics), under the title of "Production of a monochromatic, low energy positron beam using the .sup.11 B(p,n).sup.11 C reaction".
The reflection type slow positron beam generating device has been adopted because it is believed that a large amount of the fast positrons are emitted from the incident surface of the target member since most of the .beta..sup.+ decay radioisotopes are produced in the vicinity of the incident surface.
However, it is difficult with the reflection type slow positron beam generating device to continuously obtain the slow positron beam of a high intensity in the manner which will later be described.